Method of casting molten metal in coated ingot mold



United States Patent 3,220,070 METHOD OF CASTING MOLTEN METAL IN COATED INGOT MOLD Edward G. Tajkowski, Latham, N.Y., assignor to General Electric Company, a corporation of New York No Drawing. Filed Nov. 23, 1959, Ser. No. 854,562 3 Claims. (Cl. 22-192) The invention relates to the casting of ingots and more particularly is directed to the casting of molten metals intomgots.

One problem troubling the steel and other metal producing industries is the problem of producing ingots of good surface quality. When molten metal is poured into an ingot there is a tendency for the molten metal to splash against the sides of the ingot mold cavity. While a number of suggestions have been made for reducing this splashing, no complete solution has been found. Splashing is deleterious in the casting of moten metals into ingots since as the molten metal comes into contact with the relatively cool walls of the mold cavity, it freezes or solidifies. This results in inhomogeneities in the cast metal which result from inclusions in the metal corresponding to material which has been splashed onto and frozen to the ingot cavity walls. In the production of ordinary carbon steels, this inclusion problem is not of critical importance since the defective portions of the surface of the ingot can be merely cut away and remelted in a new heat. However, with more expensive alloys of ferrous and non-ferrous metals, the cost of producing the melt is quite high so that the corresponding expense caused by inclusions and surface defects is also quite high. In the case of the more expensive metal alloys, such as stainless steel and the various brasses and bronzes, which contain surface defects in the ingot, an expensive machining operation is commonly utilized to remove the surface defects to produce a homogeneous ingot which can be further processed.

In an attempt to improve the surface characteristics of castings, a number of mold washes have been tried. One of the objects of these mold washes has been to insulate the melt from the ingot mold so that freezing does not take place when the mold comes into contact with the melt. Among the many mold washes which have been tried are included paints of aluminum flake in organic binders, and tarry hydrocarbons such as Gilsonite. None of these prior art mold washes have been satisfactory because these washes either failed to prevent freezing of splashed portions of the melt onto the mold cavity or the washes have introduced impurities into the melt which have affected the composition of the melt. Thus, in those mold washes employing organic materials such as the organic binders or tarry hydrocarbons, the heat of the melt has tended to decompose the hydrocarbon resulting in carbon and hydrogen which diffuse into the surface of the melt, resulting in an ingot which is high in surface carbon or which has been embrittled by absorption of hydrogen.

It is an object of the present invention to provide an improved process for casting molten metals in ingots.

It is a further object of the present invention to provide. an improved process for the preparation of ingots having superior surface characteristics.

It is a still further object of the present invention to provide a coated ingot mold of improved characteristics for the casting of molten metals.

These, and other objects of my invention, are accomplished by coating the inside surfaces of an ingot mold with a composition comprising a fluid silicone containing a finely divided silica filler and pouring the molten metal into this coated ingot mold.

3,220,070 Patented Nov. 30, 1965 When a molten metal is poured into an ingot mold whose inside surface is treated with this fluid siliconesihca composition, molten metal droplets that are splashed up from the bottom of the cavity do not wet the walls but actually fall back into the melt. When the pouring of the melt has been completed and the ingot is removed from the mold, it is found that there are substantially no surface inclusions of prematurely frozen metal in the ingot and the ingot has a smooth surface. The surface is sometimes coated with a layer of finely divided silica which can be brushed easily from the surface of the ingot. This produces an ingot which is ready for further processing without any substantial cleaning up.

While the exact mechanism of the operation of the fluid silicone-silica composition is not completely known, it is believed that the fluid silicones either volatilize from the surface of the ingot cavity or decompose into volatile gases and silica when the mass of molten metal in the ingot comes into contact with the composition. In either case, whether the silicone fluid is decomposed or not, the silicone fluid or volatile decomposition products are swept out of the ingot mold cavity as the liquid level in the cavity rises, leaving the silica as a barrier layer between the ingot and the mold surface.

The fluid silicones employed in the practice of my invention are well known in the art and are compositions containing alternate silicon and oxygen atoms in their chain. These fluid silicones are also known as org-anopolysiloxane fluids and contain an average of about 2, for example, from about 1.95 to 2.25 organic groups per silicon atom and have a viscosity at 25 C. of from about 20 centistokes to about 100,000 centistokes. Of the fluid organopolysiloxanes known in the art, it is preferred to employ in the practice of my invention those organopolysiloxanes in which the organo groups are methyl and preferably those organopolysiloxane fluids which comprise primarily dimethylpolysiloxane fluids. These methylpolysiloxanes and the method for preparing the same are more particularly disclosed in Patent 2,469,888-Patnode.

The finely divided silica employed in the practice of the present invention can be, for example, silica aerogel, a fumed silica (prepared, for instance, by burning volatile silicon compounds such as silicontet-rachloride, tr-ichlorosilane, ethylorthosilicate, or methylorthosilicate), precipitated silicas, etc. Such fine divided silicas are partioularly described in, for example, US. Patent 2,541,137.

The ratio of fluid silicone to silica employed in the practice of the present invention can vary within extremely wide limits. However, in the preferred embodiment of my invention I employ from about 5 parts by weight to 20 parts by weight of finely divided silica per parts of the fluid silicone. When fluid silicone-silica compositions containing less than 5 parts silica, for example, about 2 parts silica per 100 parts fluid silicone, are employed in the practice of the present invention, it is found that the surface of the cast ingot is inferior to the surface of those ingots prepared by employing a silicone-silica composition in the preferred range described above.

A number of methods are available for applying the fluid silicone-silica compositions to the mold cavity surfaces. One obvious method of applying this composition is by painting the composition onto the cavity walls or by spraying the composition onto the cavity walls. Regardless of which method of application of the composition to the cavity walls is employed, the amount of the composition should be from about 0.05 to 0.5 ounce of the composition per square foot of cavity wall surface. When the fluid silicone-silica composition is applied by painting or spraying, the cavity temperature can be any 3 temperature from room temperature up to temperatures of 400 to 500 F.

While the fluid silicone-silica composition can be applied directly to the cavity wall, in the preferred embodiment of my invention, a dispersion of the composition is employed for ease of application. The most economical method of applying the composition to cavity walls is by forming an emulsion of the composition in water and spraying this aqueous emulsion onto the cavity walls. This process is of primary utility in tho e situations where the cavity walls are heated at a temperature sufficiently high so that the water in the emulsion will evaporate readily from the emulsion, leaving the cavity wall coated with the fluid silicone-silica composition. In general, it is preferred to have the ingot molds at a temperature of from about 200 to 500 F. when aqueous emulsions of the composition are employed. When the ingot mold is heated to these temperatures the water evaporates at a rapid rate leaving the desired composition as a coating on the surface of the inner walls.

Aqueous emulsions of fluid silicones containing silica which are employed in the preferred embodiment of my invention are well known in the art and are described, for example, in Patent 2,829,112Solomon and Patent 2,595,928Curn'e. The aqueous emulsions of the fluid silicone-silica compositions are readily prepared by a variety of different methods which broadly comprise the mixing of the fluid silicone, the silica and the water with a suitable emulsifying agent. One very useful method for preparing the emulsions is to wet the silica with the fluid silicone and then add the emulsifying agent to the fluid silicone-silica mixture on a colloid mill or a mixing tank. Then water is stirred in until a homogeneous mixture is formed and the entire mixture of ingredients is then passed through a colloid mill or homogenizer until a finely dispersed stable emulsion is obtained.

The emulsifying agents employed in preparing the emulsions used in the practice of the preferred embodiment of this invention are any of those well known surface active materials which are known in the art as emulsifiers. Among the many emulsifying agents which can be employed in the practice of the present invention may be mentioned, for example, glyceryl monostearate, polyoxyethylene stearate, propylene glycol monolaurate, polyvinyl alcohol, the isooctylphenyl ether of polyethylene glycol, polyoxyethylated nonyl phenol, triethanol ammonium stearate, sulfonated oils, sodium lauroyl monoglyceride sulfate, sulfonated alkyl benzenes, sulfonated alkylnaphthalenes, cetylpyridinium chloride, lauroyl diethanolamine, methyl cellulose, carboxyl methyl cellulose, etc.

While in the preferred embodiment of my invention the fluid silicone-silica composition is used in the form of an aqueous emulsion, these aqueous emulsions are not desirable for use when coating cold ingot molds with the fluid silicone-silica composition since it is necessary to remove allwater from the emulsion prior to the pouring of molten metal into the mold. Itv water is present in the coating, there is -a danger of steam explosions. For

this reason, when thevfluid silicone-silica composition is.

to be employed in coating cold mold cavity surfaces, it is desirable to employ a solution of the composition in a volatile solvent which will volatilize readily at room temperature and permit the use of the coated ingot mold shortly after the application of the fluid silicone-silica composition to its inner surface. One very satisfactory class of solvents which can be employed for thi purpose are those common commercial solvents containing chlorinated aliphatic groups such as, forexample, trichloroethylene and trichloroethane. These chlorinated compounds are particularly desirable since they are substantially fire resistant and thus almost completely eliminate the danger of fires caused by solvent. On the other hand, where no serious fire danger is produced even by those solvents which will burn, the solvents which can be employed in the practice ofthe present invention include such materials as benzene, hexane, toluene, etc. These solutions of the fluid silicone-silica compositions in solvents are prepared by merely mixing the fluid silicone with the silica and the solvent and dispersing the mixture to insure a uniform coating material.

In the case of both the aqueous emulsions and the organic solvent solutions of the fluid silicone-silica compositions of the present invention, the concentration of the fluid silicone-silica can vary from about 5 parts to 50 or more parts by weight of the total Weight of the emulsion or organic solution. No particular disadvantage is incurred in employing less than 5 percent by weight of the fluid silicone-silica mixture .but no particular advantage is gained thereby and in some cases economic considerations (waste of organic solvent) dictate the use of higher concentrations of the fluid silicone-silica composition. Where either an aqueous emulsion or an organic solution of the fluid silicone-silica composition is employed, the emulsion or solution is merely sprayed upon the surface of the ingot mold cavity in sutficient amounts to provide from about 0.05 to 0.5 ounce of the composition per square foot of mold surface after the water or organic solvent has evaporated.

The process of the present invention is applicable to the casting of almost any type of molten metal in almost any type of ingot mold. As explained previously, the process of the present invention is particularly useful in the casting of relatively expensive melts since it avoids the high waste found with casting processes where the surface of the ingot contains large amounts of inclusions.

The invention is also particularly applicable to the casting of melts into ingot molds having small CI'OSSFSBCtlOI'IS since the volume of the inclusions in this situation can be a very substantial portion of the volume of the ingot and, therefore, little useful ingot material is prepared when the surface of the ingot contains a large percentage of defects. This invention is applicable to both ferrous. and nonferrous alloys, and insofar as has been determined, is not limited at all as to composition except that the invention is primarily designed for use'in the casting of alloys which are air-melted and air-poured, as contrasted to vacuum melted and vacuum casted alloys. Among the of thepresent invention can be formed of any conven tional mold material of instruction. In most cases, for

economic reasons, ingot molds are formed of cast iron and the process of this invention isespecially useful in the casting of materials in cast iron. However, ingot molds of other materials can be employed, such as. molds of copper, brass, and molds of various other ferrous and non-ferrous alloys.

The size and shape of molds which can be employed in the practice of the present invention can also vary without limit. Thus, the present invention has. been practiced employing ingot molds which are round, square, tapered, straight, open bottom, closed bottom, solid walled, chamber walled, etc. been practiced in molds as small as cylindrical molds having a diameter of 6 inches to rectangular molds severalv feet square. The depth of these molds has also varied extensively.

The following examples are illustrative of thepracnickel-chromiurn-iron,

The present invention has.

tice of my invention and are not intended for purposes of limitation.

Example 1 30 parts by weight of a fluid silicone having a viscosity of about 100 centistokes at 25 C. and which comprised a trimethylsilyl chain-stopped dimethylpolysiloxane was mixed with 1.5 parts of a silica aerogel in such a manner that the silica was wet by the fluid silicone. Thereafter, 6 parts of the monostearic acid easter of polyethylene glycol and 8.3 parts of the monostearic acid ester of sorbitol were added to the first mixture to form a mixture of ingredients which was mixed thoroughly with 54.2 parts water until a homogeneous mixture was obtained and this. homogeneous mixture was passed through a colloid mill until a finely dispersed stable emulsion was obtained. This emulsion was sprayed by an air atomizing sprayer onto the surface of a cast iron ingot mold 22 inches square by 7 feet high in sufficient amount to provide 0.1 ounce of the fluid silicone-silica mixture per square foot of mold cavity surface. The mold was at a temperature of about 300 F. during the application of the fluid silicone-silica composition thereto. Into this treated mold cavity was poured a melt of 1020 carbon steel. During the pouring considerable splashing of the melt occurred with droplets of the melt being splashed upon the coated walls of the ingot mold. However, these droplets did not wet the coated walls of the ingot mold and immediately dropped back into the main body of the melt. After the casting of the ingot was completed, the mold was stripped from the ingot and its surface was examined. This surface was smooth and had a shiny, silvery appearance which resulted from a coating of finely divided silica. When the procedure of this example was repeated except that the inner cavity surface of the ingot mold was uncoated, droplets of the molten metal which splashed upon the mold wall instantly froze on the wall and as the level of liquid metal in the mold rose, these prematurely frozen droplets formed inclusions in the surface of the casting. These inclusions had to be ground or burned from the surface of the casting after the casting was removed from the mold. When the procedure of this example was repeated using 1030 carbon steel instead of 1020 carbon steel, the same results were obtained. That is, a smooth casting was obtained when the mold cavities were coated with the compositions employed in the practice of the present invention, whereas an ingot surface full of inclusions was obtained when no mold cavity coating was employed.

Example 2 A round ingot mold having a diameter of 6 inches and a height of 7 feet was coated with the emulsion described in Example 1 with an air atomizing spray gun to provide a coating on the interior surface of the mold of 0.05 ounce of fluid silicone-silica material per square foot of mold surface. During the spraying, the mold was at a temperature of about 250 F. and the water from the sprayed emulsion evaporated almost immediately. Into this coated mold was poured a melt of 304 stainless steel. As in Example 1, a considerable amount of splashing occurred during the pouring but droplets of metal which came into contact with the treated mold wall failed to wet the wall and dropped back into the body of the melt. Again, an ingot was obtained which was coated with a layer of finely divided silica which was readily brushed off of the ingot, resulting in a bright, shiny smooth ingot substantially free of surface defects. When the procedure of this example was repeated without any coating on the surface of the mold, the droplets of the 304 stainless steel melt which came into contact with the mold walls froze onto the walls and formed inclusion in the ingot. In addition, the ingot surface was coated with a dark gray oxide scale. The inclusions had to be burned out or ground out, resulting in a substantial waste of labor and of the expensive stainless steel alloy. When the procedure of this example was repeated with an iron titanium alloy instead of the type 304 stainless steel, the use of the fluid silicone-silica mixture again established its utility in providing an ingot free of surface inclusions.

Example 3 An aqueous fluid silicone emulsion was prepared by adding 26.2 parts by weight of a liquid polydimethylsiloxane having a viscosity of about centistokes, 2.9 parts by weight of a finely divided precipitated silica, 1.6 parts by weight of a polyethylene glycol tertiary dodecyl thioether, 0.62 part of the reaction product of a dialkyldimethyl ammonium chloride and cocoa fatty acids, and 7.6 parts by weight of a polyethylene gylcol monooleate to a ball mill and then adding 61.2 parts by weight of water to form an emulsion. This emulsion was sprayed by an air atomizer onto the cavity walls of a cast iron ingot mold 17 inches square and 6 feet high which was maintained at a temperature of about 200 F. A sufficient amount of the emulsion was sprayed to provide 0.5 ounce of the fluid silicone-silica mixture per cubic foot of cavity wall after the water had evaporated from the emulsion. When a type 316 stainless steel melt was poured into this coated ingot mold, non-wettability of the walls by the splashed molten alloy was observed. In contrast to this, when the procedure of the example was repeated except that not coating was applied to the mold wall surface, the droplets which splashed onto the mold walls during the pouring of the ingot froze onto the walls and produced inclusions in the surface of the ingot which had to be removed by burning and grinding.

Example 4 A mold wall coating material was. prepared by thoroughly mixing 10 parts by weight of a polydimethylsiloxane fluid having a viscosity of 10,000 centistokes at 25 C. with 1 part by weight of fumed silica and 89 parts by weight trichloroethylene. This material was. sprayed by an air atomizer onto the surface of a cold cast iron mold 17" x 52" x 4' high. After the trichloroethylene solvent had evaporated, a melt of type 316 stainless steel was poured into the ingot mold with the favorable results described above in connection with the nonwettabil-ity of cavity mold walls and the uniformity of the surfaces of ingots obtained.

Example 5 A mold coating composition was prepared by thorough ly mixing 10 parts of the polydimethylsiloxane described in Example 4 with 1 part of fumed silica and 89 parts of hexane. This solution was applied to the inside surfaces of a water-jacketed cylindrical copper ingot mold maintained at room temperature. After the hexane had evaporated, the copper mold Walls contained 0.05 ounce of the fluid silicone-silica composition per square foot of cavity surface. A melt of 28 percent nickel, 67 percent copper and 5 percent magnesium was poured into this ingot mold and the detailed conditions during the pouring operation were observed. The ingot melt splashed considerably and droplets which impinged upon the coated surface of the copper mold did not stick to the surface but fell back into the main body of the poured melt. After completion of the casting operation, the mold was stripped from the alloy and the surface of the ingot was carefully examined. The surface contained a very light coating of finely divided silica which was easily brushed from the surface, which was remarkably free of surface defects. When the procedure of this example was repeated without any coating applied to the copper mold surface, molten alloy droplets froze on the surface during the pouring operation and the resulting ingot was full of surface defects which made the ingot largely unsuitable for an extrusion process.

What I claim and desire to secure by Letters Patent of the United States is:

1. The method of casting a molten metal in a metallic ingot mold, which comprises coating the interior surfaces of said ingot mold with a composition consisting essentially of, by weight, 100 parts of a fluid silicone having a viscosity at 25 C. of from 20 to 100,000 centistokes and from 5 to 20 parts of a finely divided silica, said composition being employed in an amount sufiicient to provide from about 0.05 to 0.5 ounce of such composition per square foot of interior surface in said ingot mold, and thereafter pouring molten metal into the coating ingot mold.

2. The method which comprises applying an aqueous emulsion of a composition consisting essentially of, by weight, 100 parts of a fluid silicone having a viscosity of from 20 to 100,000 centistokes at 25 C. and from 5 to 20 parts of finely devided silica to the. interior walls of a metallic ingot mold, allowing the Water to evaporate therefrom to form a coated ingot mold and thereafter pouring a molten metal into said coating ingot mold, said composition being employed in an amount sufficient to provide from 0.05 to 0.5 ounce of said composition per square foot of interior wall surface in said ingot mold.

3. The method which comprises applying to the interior walls of a metallic ingot mold a dispersion in an organic solvent of a composition consisting essentially of, by Weight, 100 parts of a fluid silicone having a viscosity of from about 20 to 100,000 centistokes at 25 C. and.

from 5 to 20 parts of finely divided. silica, allowing said organic solvent to evaporate therefrom to form a coated ingot mold and thereafter pouring a molten metal into said coated ingot mold, said composition being employed in an amount sufiicient to provide from 0.05 to 0.5 ounce of said composition per square foot of surface in the interior walls of said ingot mold.

References Cited by the Examiner UNITED STATES PATENTS 1,982,763 12/1934 Russell et al. 117-5.3 X 2,473,887 6/1949 Jennings et al. 1175.3 2,573,337 10/1951 Hyde 1175.3 X 2,580,524 1/1952 Daussan 117-53 2,666,685 1/1954 Hommel et al. 106-38.22 2,755,194 7/1956 Volkmann et al. 106-3822 2,811,408 10/1957 Braley 117-5.3 X 2,955,958 10/1960 Brown 117113 3,047,917 8/1962 McCarter et al 1175.3

OTHER REFERENCES WILLIAM D. MARTIN, Primary Examiner.

RICHARD D. NEVIUS, JOSEPH B. SPENCER,

Examiners. 

1. THE METHOD OF CASTING A MOLTEN METAL IN A METALLIC INGOT MOLD, WHICH COMPRISES COATING THE INTERIOR SURFACES OF SAID INGOT MOLD WITH A COMPOSITION CONSISTING ESSENTIALLY OF, BY WEIGHT, 100 PARTS OF A FLUID SILICONE HAVING A VISCOSITY AT 25*C. OF FROM 20 TO 1000000 CENTISTOKES AND FROM 5 TO 20 PARTS OF A FINELY DIVIDED SILICA, SAID COMPOSITION BEING EMPLOYED IN AN AMOUNT SUFFICIENT TO PROVIDE FROM ABOUT 0.05 TO 0.5 OUNCE OF SUCH COMPOSITION PER SQUARE FOOT OF INTERIOR SURFACE IN SAID INGOT MOLD, AND THEREAFTER POURING MOLTEN METAL INTO THE COATING INGOT MOLD. 